Mill which operates at an overcritical speed of rotation

ABSTRACT

A mill which operates at an overcritical speed of rotation comprises a grinding container in the form of a rotational body, and one or more internal grinding members revolvable around the axis of the grinding container at an overcritical speed. The grinding member or members are rotatably mounted on a trailing arm which is mounted on a carrier that can pivot about an axis which is substantially parallel to the axis of rotation of the grinding container. An arrangement is provided for controlling the difference between the peripheral speed of rotation of the grinding member or members and the speed of rotation of the grinding container about the axis of the grinding container, and provides efficient grinding with low power consumption.

United States Patent Eirich Oct. 7, 1975 MILL WHICH OPERATES AT AN2,463,556 3/1949 Piper 241 129 OVERCRITICAL SPEED OF ROTATION L PrimaryExaminerRoy ake [75] Inventor. gilhelm Enrich, Hardhelm, AssistantExaminer E' F Desmond ermany Attorney, Agent, or Firm.Toren McGeady and[73] Assignee: Maschinenfabrik Gustav Eirich, Stanger I Hardheim,Germany 22 Filed: Mar. 11, 1974 [57] ABSTRACT A mill which operates atan overcritical speed of rota- [21] Appl' 450239 tioncomprises agrinding container in the form of a rotational body, and one or moreinternal grinding [30] F i A li ti P i it D t members revolvable aroundthe axis of the grinding Mar 14, 1973 Switzerland 3732/73 container atovercrihcal Speed- The grinding her or members are rotatably mounted ona trailing 52 us. or 241/120; 241/130 arm which is mounted carrier thatcan [51 Int. c1. 1302c 15/02 axis which is substantially Parallel the am[58] Field of Search 241/1O8 110 117, 118 of rotation of the grindingcontainer. An arrangement 241/120, [21 123, 125 126 127431 is providedfor controlling the difference between the peripheral speed of rotationof the grinding member [56] References Cited or members and the speed ofrotation of the grinding container about the axis of the grindingcontainer, and UNITED STATES PATENTS provides efficient grinding withlow power consumpl,936,593 11/1933 Fraser 241/129 X tion 2,177,94510/1939 Piper 241/118 2,303,319 12/1942 Beardsley et a1. 241/129 X 18Claims, 8 Drawing Figures us. Patent (M11975 sheetlom 3,910,509

U.S. Patent 0a. 7,1975 Sheet 2 of4 3,910,509

U.S. Patent Oct. 7,1975 Sheet 3 of4 3,910,509

US. Patent 0a. 7,1975 Sheet4 Of4 3 910509 MILL WHICH OPERATES AT ANOVERCRITICAL SPEED OF ROTATION The present invention relates to a millwhich operates at an overcritical speed of rotation, in which a grindingmember rolls over the material to be ground, in a grinding containerwhich rotates about its axis together with the material to be ground, atan overcritical speed of rotation.

When materials are being ground, in particular hard and medium'hardmaterials, the particles to be crushed are loaded until they break up,abrupt changes in force being conducive to crushing of the particles.Therefore the particles of the material to be ground should be subjectedto loadings which fluctuate as rapidly as possible, and which can beproduced by the action of pressure, impact, shearing and also due to theexpansion effect which follows the aforementioned kinds of loading. Thequite considerable power requirement necessary for crushing theparticles rises sharply with decreasing particle size. Recent findingsindicate that it is hardly possible to get below a given minimumparticle size, which varies from one material to another, because inthat region the particles can only be plastically deformed with theprevious methods, or because the amount of energy required for crushingthe particles cannot be transmitted to the particles to be crushed, orcannot be transmitted sufficiently quickly. The minimum particle size isalso determined by the particles which agglomerate under pressure.

Ball mills are extremely widely used, in'particular in the cementindustry. The degree of efficiency of such gravitational force ballmills is very low and is of the order of magnitude of 1 percent. Recenttests and attempts at increasing the level of efficiency have resultedin a considerable increase in efficiency in the case of so-called planetball mills and swing ball mills. However, hitherto these more recentlydeveloped mills could only be constructed in relatively small sizes.Although they make it possible to achieve grinding results as areobtained in much larger gravitational force ball mills, they are not ina position, due to their relative smallness, to provide the desiredincrease in respect of grinding large amounts of material.

A distinction is made between mills which have a overcritical speed ofrotation and mills which have a subcritical speed of rotation. It isknown that the critical speed of rotation in the case of mills is thatspeed of rotation at which the centrifugal force exceeds the weight ofthe members, as determined by the force of gravity.

Now if a conventional gravitational force ball mill were to be driven ata speed of rotation which is excessively overcritical, the materialbeing ground, together with the grinding balls, would remain stationary,and

there would not be any grinding action. In order to overcome this, ithas already been proposed that deflector members should be incorporatedinto the grinding container, which again locally slow the material beingground and the grinding balls down to a subcritical peripheral speed, sothat the material and the balls crash. Such mills are to a certainextent an agitator ball mill with a stationary agitator vane, whichresults in a high power requirement and relatively low speeds ofrotation. The material being ground always'has to be accelerated againto a higher speed.

2 The principle of securing the material being ground against thecylindrical walls of a grinding material container, by means ofovercritical speeds of rotation, has

also already been used in those centrifugal force mills in which acylindrical grinding member which is disposed in the grinding containerrolls overthe material which is held against the walls of the container.However, the material can only be crushed by squashing down to apredetermined grain size (corresponding to the fixed clearance betweenthe grinding member and the internal surfaceof the grinding container).A plurality of such mills are arranged one above the other,

with a vertical axis. Therefore the fact that the material to be groundis held against the walls of the container by the high centrifugalforces generated is 'utilised in this apparatus in order better to beable to crush the material. The level of success is low. The clearancebetween the grinding member and the casing of the grinding container,and thus the grain size, is varied by wear on the grindingmembers and onthe casing of y the grinding container. Success is attained only in theregion in which there is a relative movement of the material beingground, with respect to the container wall, in the form of a frictionforce, as in autogenous mills. The grinding pressure is determined bythe degree of filling of the container and the clearance between thecontainer and the grinding member, the centrifugal force being intendedonly to hold the material against the wallof the container. 7 I

In the field of rnills which operate at a subcritical speed of rotation,there is known a ball or roll mill having a vertical axis of rotation,for crushing cocoa, chocolate, varnish pigments, enamels, and the like,havinga cylindrical container within which there is arranged a coaxialrotor. Disposed at the surface of the rotor are grinding rollers whicheachhave their rotary mounting spindle guided in radially extendingslots in the rotor. Axial passages are provided in the end. walls of thecylindrical container, for the delivery and discharge of the substanceto be crushed, or the material to be ground. The material to be groundtherefore moves parallel to the axis of the container along thecylindrical wall thereof, in a downward direction, and this excludesoperation at an overcritical speed of rotation. In addition, themounting of the grinding rollers is exposed to the material to beground, so that the bearings are subjected to a high rate of wear.

Starting from this state of the art, the invention is based on theproblem of providing a grinding principle which utilises the method,which is favourable per se, of holding the material to be ground againstthe wall of the grinding container, by centrifugal force, but whichavoids thedisadvantages thereof, in particular the disadvantagesregarding degree of efficiency, wear and limitation in respect of grainsize. The invention seeks to permit grinding 'over wide ranges, withsmall, medium and large. machine units, while saving space andincreasing the degree of efficiency. It is also intended to permitsavings both as regards the apparatus and also the operating costs. I

In accordance with the invention, this problem is solved by using agrinding principle in which grinding members rotate in the grindingcontainer about the axis thereof at an overcritical speed of rotationwhich is dif-' ferent from the speed and/or the. direction of rotationof the grinding container, while the grinding members roll over thematerial to be ground, rotating about their" 3 own axis, and lyingagainst the material under the effect of centrifugal force.

By selecting suitable speeds of rotation, itis possible for the.pressure of the material to be ground, against the grinding surface,which is preferably armoured, of t the grinding container, and thepressure ofthe grinding members on the material to be ground, to reachvalues which can exceed 20 to several hundred times their weight.

The grinding principle according to the invention can be embodied bothwith substantially cylindrical grinding members alone, and also withgrinding members which in particular are in the form of wheels oftapered cross-section, in conjunction with grinding balls. Very smallballs can be used in this arrangement, which makes it possible toincrease the number of contact points and a reduction .in the contactsurface areas of the balls. When operating with grinding balls, thepower required is in fact higher than when operating without grindingballs, but normally the power required does not reach the: value of thepower required by agitator ball mills.

, A mill according to the invention is characterised in that a grindingcontainer whichis in the form of a rotational body is mounted drivablyat an ,overcritical speed of rotation about an axis which extendssubstantially parallel to its axis of rotation, and that eachgrindingmember which revolves around the axis of theygrinding container at anovercritical speed is rotatably mounted in a trailing arm which ismounted on a carrier pivotably about an axis which is substantiallyparallel to the axis of rotation of the grinding container, the carrierbeing drivably rotated about the axis of the grinding container, whilemeans are provided for controlling the difference between the peripheralspeed of rotation of the grinding members and the speed of rotation ofthe grinding container about the axis of the grinding container. V

In operation the grinding members which rotate on the pivotal trailingarms roll on the grinding face of the grinding container or the materialto be ground, with a corresponding speed difference.

The individual grinding members which are generally substantiallycylindrical can be secured to a whip-like suspension means by way ofwhich they can be drawn around the centre of the grinding container in acircular motion. As a result of centrifugal force, the grinding membersare urged towards the grinding face of the grinding container, with aforce equal to a multiple of their weight. The grinding members rollover the grinding face of the container or over the material to beground which is disposed on the grinding face, and which is compressedand held firm thereagainst by the centrifugal force. Very high levels ofcentrifugal force can be attained. The number of times that the grindingmembers roll over the material in each unit of time may be controlled byadjusting the difference in travel from ,the grinding container to thegrinding member, and by tions of rotation are the same and the speeds ofrotation are almost the same, and a maximum grinding action can beattained when the directions of rotation are opposed and the speeds ofrotation are high.

When grinding with the same directions of rotationof the grindingmembers and the grinding container, it is of advantage for the'grindingcontainer to be rotated at,

a higher speed, for example twice as fast as the grinding membercarrier. In this case the whole drive power flows through the platedrive into the machine. The

plate has a tendency to entrain the grinding members at the sameperipheral speed. However; the grinding members are kept down to thelower peripheral speed by means of the grinding member suspension means.

Thus drive power flows from the plate through the layer of material tobe ground, on the wall of the container, to the grinding members, andfrom the grinding members to the drive point of the grinding membercarrier. Therefore mechanical energy is transmitted through the layer ofmaterial to be ground, and applies a tensile loading thereto, inaddition to the high compression forces. Most materials to be groundhave a substantially lower level of tensile strength than theircompression strength. Therefore, the crushing action can besubstantially increased, with a relatively low generating grindingaction can even occur from one grain of material to another. Theexpansion effect which follows any compression effect has a similareffect on the material to be ground. Under, such conditions the rollingresistance of the grinding members is minimal.

As the rolling resistance can therefore be kept relatively low, it ispossible (in this case also the increase in weight due tocentrifugalforce is of use) for the indi-.

vidual grinding members to beef small diameters, so that very shortcompression ,zone 7 regions can be formed and the rolling speed attainedby the grinding members can be high.

In order to increase their effect the grinding members can also beeccentricallymounted and eccentrically constructed, or can be madeout-ofbalance in some other way (for example by means of weightdistribution), so that the grinding members make. it possible easily toapply the out-of-balance effect which is known from swing mills, withoutthe necessity of moving large out-of-balance masses. Owing to thepivotal mounting of the grinding members on their rotating carrier, thecontainer is substantially protected from the material to be ground.

Owing to the. high pressing forces applied by the grinding members, andtheir high peripheral speeds, when there is a sudden interruption incurrent the grinding members tend to break-out of their path ofmovement, as the tensile loading previously applied by the grindingmembers is replaced by a strong shearing action. Account should betakenof this fact when constructing the. guide elements for the grindingmembers, for example by limiting the outward movementof the grindingmembers by a mechanical abutment or a resilient means.

The grinding members preferably comprise pairs of grinding rollers, oneroller being mounted above the trailing arm and one roller being mountedbelow the trailing arm, on a shaft which is carried in the arm. In oneembodiment the shaft is non-rotatably carried in the pivotal arm, andeach grinding roller is independently rotatably mounted therein.

Small differences in the thickness of the layer of material lyingagainst the wall of the grinding container can result in varying speedsof rotation of the grinding members, so that when sudden retardationoccurs, for example due to an interruption in current, there is anincreased tendency on the part of the grinding members to veer out oftheir line of movement. Therefore, in an advantageous embodiment of theinvention, the grinding rollers of each pair are rigidly mounted ontheir shaft, while the shaft is mounted, in the form of a spindle,rotatably in the associated trailing arm. The above-mentioned danger ofthe grinding members veering out of their path of movement is overcomeby this fixed connection between the grinding rollers of each pair andthe common spindle.

The grinding members can also be constructed with an out-of-balance, inorder that the impact action or the bounce effect of the grindingmembers against the material to be ground is varied. As vibration andoscillations are produced in this case owing to the high speeds ofrotation, and these put a loading on the spindle and the bearings, itcan be advantageous for the grinding members not to be solid inconstruction, but to be so constructed in an annular configuration thata layer of resilient material is carried on an internal hub, and agrinding ring comprising a material having a high resistance to wear isfitted over the layer of resilient material. With this construction thevibration forces which occur in the grinding ring are substantiallydamped, and this construction also has the additional disadvantage thatthe grinding rings which are subjected to wear can be relatively easilyreplaced.

A further modification of the mill according to the invention providesthat the axis of rotation of the grinding member carrier is arrangedeccentrically relative to the axis of rotation of the grindingcontainer. 'When operating such a mill, the radius as between the axisof the grinding members and the axis of the container varies twice ineach revolution, that is to say, the rolling travel of each individualgrinding member is increased and shortened during one revolution, thusproducing a continuous alternate succession of acceleration and brakingat the running surface of each grinding member, so that, in addition tothe rolling and impact action of the grinding members, there is also afriction action on the material, which is conducive to grinding thematerial.

The above-mentioned incorporation of a resilient means for limiting theoutward movement of the grinding members towards the wall of thegrinding container also provides the possibility of adaptation to amaterial of a different nature, for example of a different degree ofhardness. When such a variation occurs, changing the weight of thegrinding members or the peripheral speed of the grinding member carrierinvolves expensive operations. If however an adjustable spring force isinterposed between the grinding member carrier and the grinding membermounting, for example in the respective pivotal arm, the pressure of thegrinding members against the material to be ground can be reduced orincreased, depending on the nature of the material, by adjustment to thespring force. In a practical construction a suitable traction spring canbe fitted between the trailing arm and the grinding member carrier.Alternatively a pneumatic tyre can be disposed at the periphery of thegrinding member carrier. Against the pneumatic tyre lies for example anangle lever which acts on the respective grinding member in question.All the pairs of grinding member can be loaded or relieved of load to anabsolutely uniform extent, by varying the air pressure in the pneumatictyre.

The material to be ground can also be circulated in the grindingcontainer, due to the grinding members having the effect of swirling upthe material, in a manner not dissimilar to the action of a motorvehicle wheel on an ordinary road surface. Obviously the coarserparticles in the atmosphere in the grinding container settle morequickly than the fine particles, so that the fine particles can bedischarged from the container by means of a suitably directed aircurrent. In this way it is also possible to provide a precautionaryaction (grading) by means of which the coarse particles can, so-tospeak,be concentrated the grinding container, which makes it possible .toincrease the level of efficiency of the method and the mill.

Consequently, the fine material can be discharged through a dischargechannel if the discharge apertures of the grinding container lie in theregion of a subcritical speed of rotation. If the speed of rotation inthe discharge region is overcritical, it is possible to use a collectingpipe in which a screw with a flexible shaft is movably disposed. Thisscrew is charged with material at the collecting means, and then conveysthe finely ground material through the screw tube into the vicinity ofthe mounting hub, and empties it into an annular collector. The flexiblescrew has its housing secured to the grinding member carrier, andextends through a hole in the bottom of the container. Below the bottomof the container the screw has a friction wheel drive which rollsagainst the stationary mounting hub. As the discharge end of the screwrotates with the grinding member carrier, it is necessary to provide anannular collector around the hub, in order to collect the fine materialdischarged from the container.

At least a part of the movement of air in the interior of the containercan be derived from the rotary movements of the components. It isparticularly advantageous for a blade wheel to rotate with the grindingmembers about the axis of the grinding container, whereby a desiredflowof air is produce in the container. The amount of air displaced and/orthe direction of displacement of the air can be determined by adjustmentof the blades of the blade wheel. The reduced pressure which is normallymaintained within the container, in order to avoid the formation ofdust, is usually produced by suction fans. However, the rotationalmovement of the grinding container can also be used for this purpose.

The axis of the grinding container can in principle be inclined at anydesired angle relative to the ground surface, but in particular inlarger machines the axis will preferably extend vertically or'at anangle. In that case the mounting can be at the top or at the bottom, orat both ends, or at the periphery.

Where it is desired to produce shearing forces, these can be obtained byinclining the axis of rotation of the individual grinding membersrelative to the axis of their path of rotary movement (that is to say,the axis of the 7 grinding container), although this obviously involvesan increased level of resistance to their movement.

' In principle the following steps, which canbe combined together, areavailable for controlling the pres-- sure force applied by the grindingmembers:

a. Variation in the speed of rotation and thus the cen' trifugal force,i b. Variation in the weight of the grinding members,

c. Variation in the contact surface (shape) of the e. Fitting additionalweights (acting on the grinding member shaft),

f. Controllable spring force which acts substantially radially on thegrinding members.

The above-mentioned steps (a) to leave the grinding member mountingvirtually unaffected. The above step (d) usually involves only a slightchange in loading on the grinding member mounting. In both cases this isbecause the grinding member is supported in operation against thegrinding face of the container,

by way of the material being ground. The additional weights which engagethe grinding member shaft, or which are transmitted to the grindingmember by way of the shaft mounting thereof, obviously involve anincrease ,in the loading on the mounting and the shaft.

mill. The good cooling actionalso makes it possible to carry outlow-temperature grinding.

It is possible, while the mill isoperating, to carry out a per se knownautomatic balancing operation.

The armouring on the grinding surface of the grinding container, and thegrinding members, can :be formed from composite materials, with apermanently rough surface. Grinding members of a stratified structurecan be used (plates of material of varying hardness, bonded together bya bonding agent and reinforcing).

High specific'grinding efficiency per, unit of surface area can beachieved.

The grinding members do notlift away from the ma- 'terial being ground,which can also apply in respect of balls.

Conditions are advantageous for using electrostatic and magneticpolarisation fields, and for the magnetic and pneumatic separation ofmaterial in the upper part of the grinding chamber.

Whole groups of grinding members canbe combined 7 together for mountingpurposes.

This means that the last-mentioned step is less preferred, and that,when it is not utilised, the grinding member mountings are only exposedtothe relatively minimal drive loadings, which has the effect ofreducing the amount of wear.

When using grinding balls it is possible for the move-. ment of thegrinding balls to be taken over by suitable grinding members, forexample which are in the shape of wheels of tapered cross-section. Thepower required in this arrangement is lower than in the case of agitatorball mills, but higher than in a mill without any grinding balls.Grinding balls also reduce the effective capacity of the mill and thepossible operating speed. A high peripheral speed however should have anenhancing effeet on the production of material, which can increase inproportion to the third power of the speed of rota-, tion.

The rate of wear appears similarly low to thatencountered in autogenousmills, but without the disadvantages of the autogenous mill beingencountered.

Some further advantages can be summarised as fol lows:

The rotating systems act as a stabilising gyroscope. Therefore littlenatural oscillationoccurs. The remaining oscillation effect can beprevented from affecting the ground below the mill, by suitablestructuring.

The mill can be loaded in dependence on the motor loading (sensing ofthe motor output) Not only the shape of the grinding members but alsotheir materials can be varied within wide limits. Grinding can beeffected wet or dry, with or without balls.

The fact that the material being ground lies firmly against the grindingsurface of the grinding container, under the effect of centrifugalforce, gives the best con ditions for fine grinding of the material,with a low power consumption, by rolling over the material.

The invention will now be described in greater detail by way of examplewith reference to the drawings, in

which: a

FIG. 1 shows a diagrammatic view in vertical crosssection through a millaccording to the invention,

FIG. 2 shows a diagrammatic plan view of half of the millshown in FIG.1,,

FIG. 3 shows a diagrammatic view in horizontal,

cross-section through the mill shown in FIG. 1,

FIGS. 4 to 6 show alternative embodiments of the grinding members,

FIG. 7 shows an embodiment in which the grinding member shaft is guidedand in addition the grinding member carrier rotates about an axis whichis eccentric relative to the axis of the grinding container,

. FIG. 8 is a view, similar to that of FIG. 1', of another embodiment ofthe mill. 3

Reference is firstly made in particular to FIGS. 1 to 3.

The grinding container 1 is a substantially cylindrical vessel whosecasing is provided with strengthening rings (not shown in greaterdetail) and which forms on its inward surface the grinding track 2 uponwhich the material (not shown) to be ground lies. The grinding members 3roll over the materialto be ground on .the grinding track 2, inoperation of the mill.

The grinding container 1 is rotatably mounted on the frame R on a hollowshaft member Z and is fixedly connected to a Vbelt pulley combination Swhich provides for the drive by means of a motor. The grinding containerhowever could also be driven in another manner,

for example by one or more linear motors. Itcould also be supported onair cushions.

I The central shaft A of the grinding member system extends through theshaft member Z and is supported therein. The shaft A is concentric tothe axis of rotation of the grinding container 1 and at its bottom endcarries a V-belt wheel 8 for driving the shaft A.

The shaft A has an enlarged portion A in the interior of the grindingcontainer 1, which carries rigid radial arms 30 upon which are mountedtrailing levers 32 which are pivotal about shafts 31 parallel to theshaft y A. The shafts 33 of the grinding members are mounted on thelevers 32. Thus, when the grinding member system is driven, the grindingmember system with the grinding members 3 rotates in the direction ofthe arrow K which is shown extending through the shafts 31. The greaterthe distance of the shaft 31 of each grinding member from the centre 0,and the closer the shaft is to the grinding track 2, the better can thegrinding member follow the centrifugal force which tends to press itagainst the grinding face or track 2.

The grinding container can in turn be driven in rotation either in thedirection of the solid-line arrow P, in the opposite direction tothedirection of the arrow K, or in the same direction as the direction ofthe arrow K, in the direction indicated by the broken-line arrow P. Thegrinding container and the grinding members are always driven at a speedof rotation which is substantially above the critical speed, so that thematerial (not shown) to be ground is pressed against the grinding track2, and the roller-shaped grinding members 3 are pressed against thematerial to be ground. In this case the grinding members 3 act with amultiple of their weight on the layer of material to be ground on thetrack 2, and swirl this material up. If the pressure is to be increasedwithout raising the speed of rotation, additional weights 34 can besupported for this purpose on the grinding member shafts 33, whichadditional weights 34 can be shaped in accordance with flow principles,as illustrated.

The grinding container 1 is open at the top. Its opening is covered by astationary hood 4 in which there is a pipe 40 or 40' for the supply ofmaterial to be ground into the container 1, while in the centre is apassage 41 for the supply of air. Around the passage 41 is an annuducedthrough the pipe 41. Disposed below the pipe I 41, and fixedly connectedto the shaft A of the grinding member drive arrangement, is a bladewheel 5 with adjustable blades 51. The amount of air impelled by thewheel 5 into the interior of the container 1 can be controlled byadjustment of the position of the blades 51. The blade wheel 5 couldalso be replaced by a control plate which is stationarily connected tothe rotor 41, to provide a similar function, for passive control of theair introduced into the container. By suitably extending the air supplypipe to the bottom of the container 1, it is possible for the air to beintroduced below the grinding member arms 30 so that, rising to the hood4.and .to the suction pipe 43, the introduced air is directly chargedwith fine material. In order to separate out the fine material, it maybe advantageous to operate with circulating air and dust separators, forexample cyclone separators, from which the air, even if it stillcontains some dust component, can be re-cycled back into the pipe 41.

As can be seen from FIG. 4, each grinding member is in the form of asingle large roller 30 which is mounted on a lever 30a corresponding tothe lever 30 in FIG. 3. In FIGS. 5 and 6, three grinding members 3b and30 respectively, are mounted on each lever 30b or 300 respectively. Thegrinding members 3b are of a tapered configuration, so that they permitparticularly high pressures on the material to be ground, and areparticularly suitable when used in conjunction with grinding balls. Thegrinding members 30 act upon a much smaller surface area than thegrinding member 3a, so that they also make it possible to producerelatively high pressures. They also have out-of-balance weights or.out-of-balance bores 3c, so that they each rotate individuallyeccentrically. However, as is particularly clearly shown in FIG. 3, asthe grinding members 3 in FIG. 3 are each combined in pairs and lieopposite to each other, such eccentric forces substantially compensatefor each other. This compensation of forces also makes it possible touse grinding members of different sizes from one pair to another (inFIG. 3, for reasons of space, only two pairs are shown), and suchgrinding members of different sizes roll over the material to be groundwith a different pressure and at different rolling speeds. This providesfor a variation in the grinding action.

The embodiment shown in FIG. 5 is particularly suitable for using themachine as a ball mill. In this case grinding balls of small diameterare used, which give a much greater number of point contacts thangrinding balls of larger diameter. In spite of their small size and thustheir relatively low inherent weight, because of the increase in weightproduced by the action of centrifugal force, in the present case suchballs have a substantially stronger effect than when using balls inequal size in a normal gravitational force ball mill. The speed of thegrinding member carrier must be so determined in this case that theloading is adapted to the material of the balls.

FIG. 7 illustrates the above-mentioned eccentric arrangement of thegrinding member carrier 5 relative to the shaft A of the container 1.For the sake of clarity in the drawings, the degree of eccentricity isshown in exaggerated form, whereas in actual fact it is about 5 mm, sothat the difference in the lengths of travel is from 1 to 6 percent.

FIG. 7 also shows the annular construction of grinding members, namely,the hub 3a is covered with a layer 3b of resilient material, for examplehard rubber, upon which is fitted, so as to lie closely thereagainst, agrinding ring 3c which comprises material having a high resistance towear, so that the transmission to the spindle 3a of vibration occurringin the grinding ring 30 is damped. In addition the swing arms 32 areextended beyond the rotary shaft 33a in the manner of a wagon pole 32.Rigidly mounted on the radial arm 30 is a strut 52 which ends in a forkor a slotted member 53. The pole 32' is guided in the slot of the forkor in the slotof the member 53. A traction spring 54 which connects thepole 32' to the enlarged portion A of the shaft A, as shown in'FIG. 1,ensures that the grinding member is not pressed. against the material tobe ground in such a way as-to produce a strong shearing action, forexample in the event of an interruption in current or some other mishap.The springs 54 are only shown diagrammatically in FIG. 7, but they canalso be of a different construction with an adjustable spring force, inorder in this way that the pressure of the grind ing members against thematerial to be ground can be adapted in .accordance with the conditionsof various materials to be ground.

FIG. 8 shows a mill according to the invention in which the container 1and the grinding member carrier A rotate in the same direction ofrotation, the container preferably rotating at a faster speed ofrotation. In this embodiment the-shaft A" is extended through thecontainer and the hood 4', and is mounted at its lower end in a supportframe T fitted on the frame. R. The hood 4 is sealed relative to thecontainer 1' by means of a resilient lip 55 which lies on an annularbead 56 secured to the top side of the container 1. The lip 55 is alsopressed resiliently against the bead 56 by a tube member 57 which isfilled with compressed air and which bears against a ring 58. Thissealing action is particularly suitable if the grinding operation iscarried out under a reduced pressure or in an atmosphere "oflow-temperature gas. a

When the mill operates in the same direction of rotation, with thecontainer moving at a faster speed of rotation, the container 1' tendsto entrain the grinding.

members 3 which are pressed strongly against the container. In this casethe drive of the grinding member is then transmitted in part to thegrinding members 3 by way of the layer of material to be ground, withthe result that the drive motor of the grinding member carrier operatesas a braking asynchronous generator. A part of the power suppliedtherefore flows back into the Plate diameter Plate depth carrier Speedof rotation of the grinding rings Drive motor: container Drive motor:grinding member carrier Pressure of centrifugal'force for each grindingmember,

kind can also be incorporated in the embodiment of the machine showninFlGS. l to 3, as indicated in FIG. 2.;

Finally, in order to promote the circulation of air,

ventilation ribs (not shown) can be fitted on the end faces of thegrinding members 3, so that the air flow produced in the grindingchamberin the container can with the invention are given hereinafter, but itshould.

be noted that suchdata are not given in a limiting sense, for the speedof rotation and the ratio of speed of rotation as between the containerand the grinding members can be extensively varied according to theconditions of grinding. All the values given hereinafter relate to a'container. and grinding members rotating. in opposite directions. Whenthe. container and the grinding members rotate in the same direction,the relative speed can be greatly reduced, while thepressure applied tothe material to be ground is maintained at the level of pressureobtaining in the. counter-running mode of operation.

1400 mm 500 mm 300 mm 100 mm l 10 kg approximately I00 400 r.p.m. aboutI00 400 r.p.m.

at I00 r.p.m.: 933

400 r.p.m. 3732 I500 r.p.m. 90 kW 1500 r.p.m. 90 kW comprising 2rollers, is, at a speed of rotation I00 for the grinding member carrier:

for the grinding member carrier: Peripheral speed of container at I00r.p.m.

Peripheral speed of the grinding member carrier Peripheral speed of thegrinding member rollers kg 740 at the speed of rotation 400 kg I2000 7.3m/sec at 400 r.p.m. 29.0 m/sec at I00 r.p.m. 5.5 m/sec at 400 r.p.m.22.0 m/sec at I00 r.p.m. 14.6 m/sec. at 400 r.p.m. 58.5 m/sec Number oftimes that all points on the grinding surface of the container arerolled over:

at I00 r.p.m. 800 per minute I3 per second at 400 r.p.m. 3200 per minute53 per second current network. As shown in the drawing, this grindingoperation can be carried out with an even greater saving of power if anelectric motor 59 with two shaft ends 60 and 61 is used for the wholedrive for the machine. As in the embodiment shown in FIG. 1, the shaftend 60 drives the container 1, while the second shaft end 61 isconnected by way of the V-belt wheel S to the grinding member carrier A,and acts as'a brake. The power which is put out by way of the grindingmember carrier A is therefore supplied to the same motor shaft, so thatthere is no necessity for conversion into electrical power.

FIG. 8 also shows a discharge channel 62 for the fine material. Thechannel 62 .is connected to a hood 63 with a downwardly directedaperture, the hood 63 being secured to the grinding member carrier A andconsequently rotating therewith. The hood 63 extends through a circularopening in the bottom of the container. l, and its end opens into astationary hopper 64 which encloses the shaft A" and into which theground fine material is discharged. A discharge channel of this Thesefigures show very remarkable average performances. However, they onlyprovide some datum points. a i

The invention is equally well suited for the construction of small,medium and large mills, for high levels of throughput are achieved withrelatively low weight and building material cost, and a relatively smallamount of space.

What I claim is: 1. A, mill comprising a. a grinding container arrangedto contain material to be ground and having an axis ftherein about whichthe container is drivably rotatable at an overcritical speed of rotationand an internal cylindrical material grinding face,

b. a grinding member carrier positioned within the I grinding containerand rotatable therein at an overd. grinding members rotatably mounted onsaid trailing arms within said container inwardly from said grindingface at a position on said trailing arms spaced from the point ofpivotal attachment to said carrier,

e. means for controlling the difference between the peripheral speed ofrotation of the grinding members and the speed of rotation of thegrinding face of said grinding container, so that the grinding membersare forced outwardly by centrifugal force to roll over the material tobe ground which is distributed within the container over the materialgrinding face by centrifugal force of said container, and the normalspeeds of rotation of the grinding container and the grinding membersare in a range in which the centrifugal forces in the region of thematerial to be ground and at the grinding member are a multiple of theirweight.

2. A mill according to claim 1, wherein the pivot axes of the trailingarms are at a smaller spacing from the material grinding face than thegrinding member diameter. f

3. A mill according to claim 1, wherein grinding members of identicalconfiguration are mounted diametrically opposite each other with respectto the axis to form pairs of grinding members and there are differencesof configuration as between said pairs;

4. A mill according to claim 1, wherein at least one grinding member isout-of-balance.

5. A mill according to claim 1, wherein the normal speeds of rotation ofthe grinding container and the grinding members are in a range in whichthe centrifugal forces in the region of the material to be ground and atthe grinding members have a value which exceeds to several hundred timestheir weight.

6. A mill according to claim 1, wherein at least one grinding member ismounted on a shaft which is inclined with respect to the axis of thegrinding container.

7. A mill according to claim 1, comprising pairs of disc-shaped grindingmembers of conical cross-section, and grinding balls arranged betweenthe grinding discs.

8. A mill according to claim 1, comprising air impeller bladespositioned within and arranged to rotate within the grinding container.

9. A mill according to claim 1, wherein the grinding container and thegrinding member carrier are arranged to rotate in the same direction andthe grinding 14 container rotates at a higher speed of rotation wherebydrive power flows from the grinding container through the layer ofmaterial being ground, to the grinding members, and from there to thegrinding member carrier.

10. A mill according to claim 9, comprising a single drive motor withtwo shaft ends, of which one shaft end drives the container andthesecond shaft end is connected to the grinding member carrier wherebythe grinding member carrier is braked.

11. A mill according to claim 1, wherein the movement of the grindingmembers in an outward direction under the effect of centrifugal force islimited by a mechanical abutment.

12. A mill according to claim 11, wherein the movement of the grindingmembers in an outward direction under the effect of centrifugal force islimited by resilient means. 7

13. A mill according to claim 12, wherein a spring for limiting theoutward movement of the grinding members is constructed with anadjustable spring force.

14. Apparatus according to claim 1, wherein the axis of rotation of thegrinding member carrier is arranged eccentrically with respect to theaxis of rotation of the grinding container.

15. A mill according to claim 1, comprising discharge means from saidcontainer for the fine material produced in the grinding operation,which discharge means rotates with the grinding member carrier..

16. A mill according to claim 15, wherein the discharge means comprisesa substantially radially extending discharge channel whose outletaperture co-operates with anannular stationary hopper which surroundsthe axis of the grinding container.

17. A mill according to claim 1, wherein the grinding members comprise aplurality of rings having hubs which carry at least one layer ofresilient material and grinding rings comprising a material which has ahigh resistance to wear comprises the grinding surface of the members.

18. A mill according to claim 1, wherein the grinding members comprisepairs of grinding rollers,-in which the grinding rollers are rigidlymounted on both sides of the centre of their axis which is formed as aspindle, and the spindle is rotatably mounted at its centre part in theassociated trailing arm.

1. A mill comprising a. a grinding container arranged to containmaterial to be ground and having an axis therein about which thecontainer is drivably rotatable at an overcritical speed of rotation andan internal cylindrical material grinding face, b. a grinding membercarrier positioned within the grinding container and rotatable thereinat an overcritical speed, c. a plurality of trailing arms pivotallymounted on said carrier within said container and extending from thepoint of pivotal attachment in a direction opposite to the rotationaldirection of said carrier, d. grinding members rotatably mounted on saidtrailing arms within said container inwardly from said grinding face ata position on said trailing arms spaced from the point of pivotalattachment to said carrier, e. means for controlling the differencebetween the peripheral speed of rotation of the grinding members and thespeed of rotation of the grinding face of said grinding container, sothat the grinding members are forced outwardly by centrifugal force toroll over the material to be ground which is distributed within thecontainer over the material grinding face by centrifugal force of saidcontainer, and the normal speeds of rotation of the grinding containerand the grinding members are in a range in which the centrifugal forcesin the region of the material to be ground and at the grinding memberare a multiple of their weight.
 2. A mill according to claim 1, whereinthe pivot axes of the trailing arms are at a smaller spacing from thematerial grinding face than the grinding member diameter.
 3. A millaccording to claim 1, wherein grinding members of identicalconfiguration are mounted diametrically opposite each other with respectto the axis to form pairs of grinding members and there are differencesof configuration as between said pairs.
 4. A mill according to claim 1,wherein at least one grinding member is out-of-balance.
 5. A millaccording to claim 1, wherein the normal speeds of rotation of thegrinding container and the grinding members are in a range in which thecentrifugal forces in the region of the material to be ground and at thegrinding members have a value which exCeeds 20 to several hundred timestheir weight.
 6. A mill according to claim 1, wherein at least onegrinding member is mounted on a shaft which is inclined with respect tothe axis of the grinding container.
 7. A mill according to claim 1,comprising pairs of disc-shaped grinding members of conicalcross-section, and grinding balls arranged between the grinding discs.8. A mill according to claim 1, comprising air impeller bladespositioned within and arranged to rotate within the grinding container.9. A mill according to claim 1, wherein the grinding container and thegrinding member carrier are arranged to rotate in the same direction andthe grinding container rotates at a higher speed of rotation wherebydrive power flows from the grinding container through the layer ofmaterial being ground, to the grinding members, and from there to thegrinding member carrier.
 10. A mill according to claim 9, comprising asingle drive motor with two shaft ends, of which one shaft end drivesthe container and the second shaft end is connected to the grindingmember carrier whereby the grinding member carrier is braked.
 11. A millaccording to claim 1, wherein the movement of the grinding members in anoutward direction under the effect of centrifugal force is limited by amechanical abutment.
 12. A mill according to claim 11, wherein themovement of the grinding members in an outward direction under theeffect of centrifugal force is limited by resilient means.
 13. A millaccording to claim 12, wherein a spring for limiting the outwardmovement of the grinding members is constructed with an adjustablespring force.
 14. Apparatus according to claim 1, wherein the axis ofrotation of the grinding member carrier is arranged eccentrically withrespect to the axis of rotation of the grinding container.
 15. A millaccording to claim 1, comprising discharge means from said container forthe fine material produced in the grinding operation, which dischargemeans rotates with the grinding member carrier.
 16. A mill according toclaim 15, wherein the discharge means comprises a substantially radiallyextending discharge channel whose outlet aperture co-operates with anannular stationary hopper which surrounds the axis of the grindingcontainer.
 17. A mill according to claim 1, wherein the grinding memberscomprise a plurality of rings having hubs which carry at least one layerof resilient material and grinding rings comprising a material which hasa high resistance to wear comprises the grinding surface of the members.18. A mill according to claim 1, wherein the grinding members comprisepairs of grinding rollers, in which the grinding rollers are rigidlymounted on both sides of the centre of their axis which is formed as aspindle, and the spindle is rotatably mounted at its centre part in theassociated trailing arm.